A single clause of the form symbol=expression is an instruction known as an assignment. An assignment gives a (new) value to a variable. See Section 1.13, “Assignments and Symbols”.
1.12.5.1. Extended Assignments
The character sequences +=, -=, *= /=, %=, //=, ||=, &=, |=, and &&= can be used to create extended assignments. These sequences combine an operation with the assignment. See Section 1.13.1, “Extended Assignments” for more details.
